/Engine/Source/Runtime/SlateCore/Public/Fonts/FontCache.h
The index used here is relative to the start of the text we were shaped from, even if we were only shaped from a sub-section of that text
The kerning, or an unset value if we couldn't get the kerning (eg, because you specified a merged ligature, or because the index is out-of-bounds)
[TOptional](API\Runtime\Core\IO\TOptional)< int8 > GetKerning ( const int32 InIndex ) const